Recent industrial demand in the area of microwave filters clearly indicates a need both for higher frequencies of operation and reduced manufacturing cost and time. In support of these needs, there is an on-going very substantial research and development effort aimed at producing faster and more versatile CAD (computed-aided design) packages for microwave filter design.
As a result of this activity, it is now possible, in some cases, to manufacture electrical filters in "one single run" going directly from conception to manufacture without breadboard or post-manufacture tuning. Although the use of advanced CAD packages is a clear benefit, there is still the problem that in order to obtain good results in one single run, the hardware must be manufactured with high accuracy. It is a well-known fact, in the field of interest, that the higher the manufacturing accuracy the higher are the manufacturing time and cost of the hardware and thus some of the desired benefit is lost.
More specifically, currently available electrical filters, for both ground and space applications mainly belong to the Chebycheff family, (with or without transmission zeroes). The filters operating in the microwave range of common interest are usually ones having waveguide type structure. Recent work on this type of electrical filter has shown that a critical factor in the achievement of a "one single run" result, without the need for post-manufacture tuning, is the relative separation in frequency of the return loss zeroes. These findings are disclosed by Marco Guglielmi and Graham Connor in a paper entitled "Industrial Implementation of Tuning-Less Microwave Filters", published in "Microwave Engineering Europe", December, January 1996, pp. 39-40. The filters discussed in this document are based on thick inductive windows implemented in a rectangular waveguide.
In standard Chebycheff filters, the return loss zeroes are equispaced within the filter band-pass thus providing an equiripple response. However, if one of the resonators the filter is comprised of or if one of the couplings of the filter does not have the required dimensions due to a manufacturing error, the electrical performance of the filter is adversely affected.
Another family of filter which is known is the Butterworth family. In this type of filters, all of the return loss zeroes are located in the center of the band-pass. The result is that this type of filter is less sensitive than a Chebycheff filter to manufacturing errors but still, it exhibits a continuously curved response throughout the band-pass. This feature makes Butterworth filters not suitable for many practical applications.